Pedicle screws basically serve the dorsal stabilization of the vertebral column in case of fractures, tumors, inflammations, deformities and degenerative instabilities by means of a transpedicular screw connection system. In this arrangement, pedicle screws are placed in the pedicles of neighboring vertebrae, creating an angularly stable connection between the pedicle screws arranged axially one upon the other and an axially extending longitudinal beam or web. In such arrangement, the pedicle screws and the longitudinal beam form a vertebral stabilizing system.
To this end, a pedicle screw usually has an axial, shaft-like external thread portion adjoined by a so-called tulip at the side of the screw head. The structure of said tulip forms a U-shaped mounting sleeve which is slotted/tunneled in the longitudinal direction and comprises an internal thread, with the two opposite longitudinal slots each defining a slot gap with a predetermined gap width. The longitudinal beam is inserted in the longitudinal slots extending parallel to each other and is fixed by means of a grub screw or threaded nut which is screwed in the internal thread.
In principle, pedicle screws are grouped in two basic types, namely mono-axial and polyaxial pedicle screws. In the case of a mono-axial pedicle screw, the external thread portion and the tulip are formed in one piece with each other such that they are firmly connected to each other, for instance welded or soldered. A polyaxial pedicle screw, however, comprises an external thread portion which is manufactured as a separate shaft piece and has a mostly ball-shaped or (semi) spherical screw head which is encompassed by the sleeve-shaped tulip in a relatively pivotable manner and at the same time engaged by the latter in the transition area between head and shaft. In this way, the tulip can be swiveled and/or rotated relative to the shaft after having lowered the external thread portion in the pedicle channel of a vertebra, in order to obtain a desired position and alignment substantially regardless of the alignment relative to the shaft. Here, the undercut prevents the tulip from being pulled off the shaft head. Subsequently, the tulip is fixed in its position on the screw head by means of the grub screw with an interposed web (single-screw principle) or through an additional screw/screw nut (multi-screw principle).
The prior art discloses, for instance according to EP 2 301 458 A1, a polyaxial pedicle screw according to the single-screw principle consisting of a shaft piece comprising an external thread and a spherical head as well as a U-shaped mounting sleeve (tulip) provided with a longitudinal slot and intended for receiving a longitudinal beam/web. In the axial area toward the opening of the longitudinal slots, the mounting sleeve has an internal thread for screwing in a grub screw, and in the axial area toward the respective slot base it comprises a circumferential protrusion or shoulder with radially inward orientation. Further, a sort of piston or punch (also referred to as an inlay) is inserted in the mounting sleeve so as to be axially shiftable therein and prevented from falling out by means of a snap ring.
For assembling the polyaxial pedicle screw known from EP 2 301 458 A1, the mounting sleeve is first slipped over the shaft piece starting from the distal end thereof (the end which is opposite the shaft head) until the radially inward shoulder of the mounting sleeve abuts against the shaft head (at its underside). Subsequently, the punch will be pressed into the mounting sleeve (above the shaft head), so that the snap ring circumferentially arranged between the mounting sleeve and the punch snaps in place in corresponding circumferential grooves on the punch and on the mounting sleeve and retains the two parts axially next to each other. Thus, the shaft head is situated between the shoulder and the punch (i.e. underneath the punch).
As soon as the pedicle screw has been screwed in a vertebra and firmly anchored therein, a longitudinal beam is inserted in the U-shaped (double) slot of the mounting sleeve (above the punch), with the possibility that the mounting sleeve can rotate and swivel relative to the anchored shaft piece. This allows a surgeon to adapt the mounting sleeve in accordance with the alignment of the longitudinal beam. As soon as the suitable relative position of the mounting sleeve is adjusted, the grub screw is screwed into the mounting sleeve until it makes the longitudinal beam contact the punch and presses the latter farther in the axial direction of the mounting sleeve against the shaft head. In this way, the entire system made up of the pedicle screws and the longitudinal beam (vertebral stabilizing system) can be fixed in the adjusted position by tightening the single grub screw.
The document US 2011/0046683 A1 discloses e.g. a polyaxial pedicle screw according to the multi-screw principle. This pedicle screw, too, has a shaft-like external thread portion comprising an integral shaft head at a proximal end of the shaft. The shaft head is surrounded by a freely rotatable and pivotable mounting sleeve which is likewise provided with an internal thread and comprises two U-shaped opposing longitudinal slots for a longitudinal beam.
A piston/punch (inlay) is inserted in the mounting sleeve so as to be axially shiftable therein and is likewise provided with a U-shaped longitudinal slot having approximately the same slot width dimensions as the longitudinal slots in the mounting sleeve.
For the purpose of assembling the pedicle screw known from US 2011/0046683 A1, the mounting sleeve/tulip is slipped over the shaft in a known manner until it rests axially at the shaft head (at the underside) against a radially inward sleeve shoulder in pivotable and rotatable manner. As a next step, the punch (above the shaft head) is inserted in the mounting sleeve and its U-shaped slot is aligned in accordance with the U-shaped slots in the mounting sleeve. A first screw/screw sleeve will then be screwed in the mounting sleeve, directly acting on the punch (inlay) in order to press the latter against the shaft head, if necessary. This first screw/screw sleeve has an internal thread into which a second screw/grub screw is screwed which exerts a compressive force on a web/longitudinal beam transversely inserted in the longitudinal slot of the mounting sleeve and of the punch, to clamp the beam against the punch.
If the pedicle screw according to US 2011/0046683 A1 is to be inserted in a vertebra, the shaft is screwed in the vertebra and then the angular orientation of the mounting sleeve is aligned relative to the screwed-in shaft. In order to fix the position of the mounting sleeve, the first screw will then be tightened and presses the punch (inlay) directly against the shaft head and thereby braces the mounting sleeve with the shaft head. Finally, a longitudinal beam is placed in the longitudinal slot between the shaft head and the first screw in transverse orientation and is clamped against the punch by means of the second screw. Consequently, the advantage of this pedicle screw according to US 2011/0046683 A1 is that the processes of fixing the polyaxial support and the longitudinal beam are carried out independently of each other, but a comparably complicated screw design is required here.
Furthermore, all the mentioned pedicle screws have their fixing devices/locking elements (screws) realized in a substantially self-locking design, so as to avoid the risk of an unwanted detachment of the longitudinal beam from the pedicle screws after implantation. Further, the fixation forces between the pedicle screw and the longitudinal beam are large, as the entire system has to withstand high loads providing that the adjusted positional relation between the shaft, the sleeve and the longitudinal beam does not change. These requirements, however, cause problems during the implantation process.
If it has happened that a surgeon has tightened the locking element (for example the grub screw) with force, the resultant positional relation between the mounting sleeve and the shaft and/or between the mounting sleeve and the longitudinal beam cannot be altered any more or only with large effort. In other words, the surgeon would have to unloose the screw(s) (which has/have been tightened with high force) against the self-locking effect, without detaching the external thread portion already anchored in the vertebra or even breaking it out. Further, any subsequent process of unloosing the locking element (screw) might have a negative impact on its self-locking effect, so that the functionality of the pedicle screw as a whole is not ensured any more. This is why the known systems are not fault-tolerant or only to a limited extent.